In the field of horology, the oscillating mechanical system may be a balance on which a balance spring is mounted, one end of which is secured to the rotating balance staff and the other end of which is secured to a fixed element of a bottom plate. The oscillation of the mechanical system is maintained via an energy source which is generally mechanical. This energy source may be, for example, a barrel driving a gear train with an escape wheel cooperating with a pallet lever. This rotating pallet lever for example actuates a pin secured in proximity to the rotating balance staff. The balance with the balance spring may thus form a regulating member of a timepiece movement. This oscillating regulating member determines the driving speed of the gear train with the escape wheel leading to the time indicator hands.
In order to precisely regulate the oscillation frequency of the oscillating mechanical system, the length of the spring may be adapted or a weight may be added or removed from the external circular part of the balance. However in the case of a wristwatch, all these additional regulating elements occupy considerable space inside the watch case, and lead to relatively long manufacturing time and high cost. This thus constitutes drawbacks.
In a mechanical or electromechanical watch, it is known to regulate the rotational speed of an electric generator connected to the spiral-shaped spring barrel to mechanically drive the hands of the watch via a gear train. The electric generator generates an alternating voltage, which is rectified by means of a rectifier of an electronic regulating circuit. The function of this regulating circuit is to control the rotational speed of the generator so that the time indicator hands can be moved as a function of the correct current time indication. A transistor of the regulating circuit can short-circuit the generator at determined time periods so as to brake the generator and thus regulate the rotational speed. EP Patent Application Nos. 0 762 243 A1 or 0 822 470 A1, which disclose a watch provided with this type of regulating circuit may be cited in this regard.
The aforementioned electric generator includes rotating permanent magnets and a coil opposite the magnets, capable of providing an induced alternating voltage. It can be complicated to make this type of generator and regulating circuit. A large number of elements generally have to be provided to design said generator with the regulating circuit. Moreover, the magnetic field of the rotating magnets may cause interference in some nearby ferromagnetic parts. This therefore constitutes several drawbacks.
Instead of an electric generator formed of rotating permanent magnets and a coil generating an induced alternating voltage, FR Patent No. 2 119 482 has already proposed providing an oscillating mechanical system with a piezoelectric element. This piezoelectric element is preferably arranged on a balance spring connected to a balance. To achieve this, films of piezoelectric material (PZT) are deposited on most of the length of the spring and on an inner face and outer face of said metal spring. A voltage converter supplies an alternating voltage to the piezoelectric element to alternately generate a compression force and an extension force on the spring in order to regulate the oscillation of the balance connected to the balance spring. However, in this Patent document, no mention is made of regulating the oscillation frequency of the balance with the balance spring using an autoregulating circuit, which is a disadvantage.
Regulating the oscillation frequency of a balance combined with a piezoelectric balance spring as the alternating voltage generator is known from JP Patent Application No. 2002-228774. The alternating voltage is rectified in a rectifier which comprises at least two diodes and FET transistors controlled by the electronic regulating circuit. The rectified voltage is stored at least in a supply voltage storage capacitor. The electronic circuit may be powered directly by the alternating voltage from the generator, which has been rectified and stored in the capacitor. The piezoelectric generator is of the bimetal type (PZT). To regulate the oscillation frequency, a comparison is made between a signal at a reference frequency supplied by a quartz oscillator circuit and the alternating signal from the generator. With the proposed electronic circuit it is not possible to devise a very compact oscillating mechanical system with the regulating circuit in a manner that is easily accomplished, which constitutes a drawback.